An international research team from the International Ocean Discovery Program (IODP), a scientific deep ocean drilling program in which the Alfred Wegener Institute (AWI) is involved, has found that the West Antarctic Ice Sheet retreated far inland at least five times during past warm periods. In a new study, the researchers analyzed geochemical signatures of deep-sea sediments from the Amundsen Sea. The results highlight the sensitivity of the ice sheet to warming and its potential to contribute to future sea level rise. The study was published in the journal PNAS.
The Thwaites and Pine Island glaciers in the Amundsen Sea sector of the West Antarctic Ice Sheet (WAIS) are among the fastest melting glaciers on Earth - together they are losing ice faster than any other part of Antarctica. This could affect the long-term stability of the ice sheet and contribute significantly to sea level rise. To better understand the risks that warmer conditions pose to the WAIS, researchers from the International Ocean Discovery Program are looking back to the Pliocene epoch (5.3 to 2.58 million years ago), when global temperatures were around 3 to 4°C higher than today. Sea levels were more than 15 meters higher than today and melted ice from Antarctica contributing to much of that rise.
“The history of the WAIS, especially in the Pliocene, was one of the most important drilling targets of the IODP expedition, because the warm periods of this epoch are comparable to future climate scenarios with expected similarly high global temperatures,” says Prof. Karsten Gohl (AWI), one of the two expedition leaders and co-author of the study.
In a new study, the IODP has analyzed deep-sea sediments recovered from the Amundsen Sea. They are a kind of historical archive that records the changes in ice sheets and ocean conditions over millions of years. Prof. Keiji Horikawa from the Faculty of Science at the University of Toyama led the study: “We wanted to investigate whether the WAIS fully disintegrated during the Pliocene, how often such events occurred, and what triggered them.”
The results show that the WAIS margin retreated at least five times far inland during the Pliocene, which illustrates its vulnerability to warming. The researchers identified two distinct sediment layers reflecting alternating cold and warm climate phases: thick, gray, and finely laminated clays from cold glacial periods, when ice extended across much of the continental shelf; and thinner, greenish layers formed during warmer interglacial periods. The green color comes from the microscopic algae, indicating open, ice-free ocean waters. Crucially, these warm-period layers also contain iceberg-rafted debris (IRD), small rock fragments carried by icebergs, that broke off from the Antarctic continent. As these icebergs drifted across the Amundsen Sea and melted, they released this debris onto the seafloor. Based on these traces in the sediments, the team found 14 distinctive intervals between 4.65 and 3.33 million years ago, in each of which major melting events took place and the WAIS partially retreated.
To determine how far inland the ice had retreated, the researchers analyzed the chemical “fingerprints” of the sediments. They measured isotopes of strontium, neodymium, and lead, which vary depending on the age and type of the source rock. By comparing these signatures with those of modern seafloor sediments and bedrock samples from across West Antarctica, the team traced much of the debris to the continental interior, particularly the Ellsworth-Whitmore Mountains.
“Our data and model results suggest that the Amundsen Sea sector of the WAIS persisted on the shelf throughout the Pliocene, punctuated by episodic but rapid retreat into the Byrd Subglacial Basin or farther inland, rather than undergoing permanent collapse,” says Keiji Horikawa. This emphasizes its extreme vulnerability to future warming and its potential to drive substantial sea-level rise.
Original publication
K. Horikawa, M. Iwai, C. Hillenbrand, C.S. Siddoway, A.R. Halberstadt, E.A. Cowan, M.L. Penkrot, K. Gohl, J.S. Wellner, Y. Asahara, K. Shin, M. Noda, M. Fujimoto, & Expedition 379 Science Party, Repeated major inland retreat of Thwaites and Pine Island glaciers (West Antarctica) during the Pliocene, Proc. Natl. Acad. Sci. U.S.A. 123 (1) e2508341122, https://doi.org/10.1073/pnas.2508341122 (2026).